Crack Paths 2012

ΔKI(A)

4

ΔKI(B)

τ

τ

Crack

ΔKIII

branching

b

Axial direction b

τ

2a II

2

τ

a

I

Δ K

I(B )

Δ K II

45°

Δ K

III

Δ K

I(A)

230

15õ

16õ

17õ

18õ

19õ

Number of cycles, (cycles) N

[10]×

(b) Crack A

(a) Definition

ΔKI(NA)umber of cycles, (cycles) N

[10]×

30 12

30

Number of cycles, (cycles) N ΔΔKΔKIKI(IABI)

[×104]

9

10

11

(c) Crack B

(d) ack C

ΔKΔKIKI(IBI)

Crackbranching

11

120

9

10

4

Figure 12. Change of stress intensity range.

coalesced at N=9.90×104 cycles. Figure 11 (b) indicates that the crack growth rate in

the depth direction was very fast just before and after the coalescence. After the crack

coalescence, the crack branching was observed both at and under the surface. The crack

branching is considered to be triggered by the coalescence.

The stress intensity factor ranges, ΔKI, ΔKII, and ΔKIII, are plotted against the number

of cycles in Figure 12, where ΔKI(A) and ΔKII are the stress intensity factors at the crack

tip at the surface, and ΔKI(B) and ΔKIII are the stress intensity factors at the depth of the

crack tip. The stress intensity factor range, ΔKII and ΔKIII were calculated by using

Kaissir and Sih’s equation [10] based on the crack shape projected to the plane parallel

to the specimen axis. The values of ΔKI(A) and ΔKI(B) were calculated by using Irwin’s

equation [11],[12] based on the crack shape projected to the principal plane of stress, as

120